Clonal composition and persistence of antigen-specific circulating T follicular helper cells.

T follicular helper (T ) cells play an essential role in promoting B cell responses and antibody affinity maturation in germinal centers (GC). A subset of memory CD4 T cells expressing the chemokine receptor CXCR5 has been described in human blood as phenotypically and clonally related to GC T cells. However, the antigen specificity and relationship of these circulating T (cT ) cells with other memory CD4 T cells remain poorly defined. Combining antigenic stimulation and T cell receptor (TCR) Vβ sequencing, we found T cells specific to tetanus toxoid (TT), influenza vaccine (Flu), or Candida albicans (C.alb) in both cT and non-cT subsets, although with different frequencies and effector functions. Interestingly, cT and non-cT cells specific for C.alb or TT had a largely overlapping TCR Vβ repertoire while the repertoire of Flu-specific cT and non-cT cells was distinct. Furthermore, Flu-specific but not C.alb-specific PD-1 cT cells had a "GC T -like" phenotype, with overexpression of IL21, CXCL13, and BCL6. Longitudinal analysis of serial blood donations showed that Flu-specific cT and non-cT cells persisted as stable repertoires for years. Collectively, our study provides insights on the relationship of cT with non-cT cells and on the heterogeneity and persistence of antigen-specific human cT cells.